Use of Nephelometric Turbidity to Calculate Carlson's Trophic State Index in Keystone Lake Oklahoma

The relationships among measurements of turbidity and chlorophyll and phosphorus were examined at six stations in Keystone Lake, Oklahoma from March to October 1981. Maximum chlorophyll a, total phosphorus, and turbidity occur red in the upper reaches of the two main arms. It was not possible to predict adequately chlorophyll a from Secchi disc readings due to interference by non-chlorophyll turbidity. Secchi disc depth represented 30 to 50% of the true photic zone and these two parameters were well correlated. Photic zone measurements were poorly correlated with phosphorus and chlorophyll a. Nephelometric turbidity was strongly correlated with all parameters except chlorophyll a. Carlson strophicstate index was of limited usefulness due to the weak relationship among parameters such as chlorophyll a and Secchi disc depth. Calculation of this index based on nephelometric turbidity strengthened the relationship.